1. Field of the Invention
This invention relates generally to the measurement of the concentration of solids in a slurry and, more particularly, to a method of and apparatus for measuring the volumetric concentration or percent solids in a slurry flowing in a large diameter pipeline.
2. Description of the Prior Art
The transportation of particulate solids in a liquid slurry is widely used in many industries, and maintaining the concentration of solids in the slurry within relatively narrow limits is essential to successful and economic operation of such systems. For example, when the concentration of solids becomes too high, excessive precipitation or settling can plug the pipeline, making it necessary to shut down and clean the system. Conversely, the cost of operating the slurry pumps is a major consideration and maintaining the concentration as high as practical while avoiding plugging is desirable for economic operation. It is, therefore, desirable to carefully monitor the concentration of the slurry in order to operate such a system near optimum conditions.
Numerous systems have been proposed and developed for measuring particulate concentration in flowing slurry suspensions. For example, the use of radiation attenuation, ultrasonics, microwave resonance, capacitance, conductance, laser, radar, light scattering and light attenuation have been proposed. However, for high concentration slurries in large pipes which may typically be 20 inches or greater in diameter such as those used in mining operations, some of these proposed systems are inoperable or impractical. For example, in such large pipes, light techniques are not usable since the slurry is essentially opaque to radiation in the visible spectrum. Also intrusive sensors are generally considered impractical since the abrasive nature of the slurry quickly erodes and destroys the sensor. As a result, radiation attenuation techniques using gamma rays or x-rays are frequently used, but they may present a radiation hazard to workers and, therefore, the monitors generally must be placed at a location not easily accessible to operating personnel.
A device for measuring the concentration of the slurry moving in a pipeline, using gamma radiation detection techniques, is disclosed in U.S. Pat. No. 5,008,906. This patent generally discusses the configuration and operation of such devices, but does not address the problem of radiation generally associated therewith. Slurry concentration measuring or monitoring devices such as those discussed in this patent are widely used, for example, in phosphate mining operations in which slurry pipelines are used to transport raw materials to the processing plant. The practice generally has been to locate the gamma ray densitometer near the end of the pipeline or in the processing plant in a remote location not routinely accessible to mine operators. In such an installation, the water jet operator who continuously mixes the slurry does not know the slurry concentration until it reaches the densitometer which may involve a delay time of up to an hour or more. Thus, since the gamma ray densitometer readout does not provide an instantaneous indication of the concentration of the slurry being mixed, it is conventional practice to rely upon the jet operator's experience and the pump motor amperage as an instantaneous indicator of slurry concentration at the pump discharge. This practice can lead to costly shut-downs due to plugging or to inefficiencies due to low concentration.
The measurement of slurry solids content by electrical conductivity is discussed by Holdridge et al in an article entitled "Measurement of Slurry Solids Content by Electrical Conductivity" published in 72 Power Technology at pages 77-87 (1992). Their tests evaluated sedimentation and flowing slurries of various solids in a conducting fluid, i.e., water, utilizing electrical conductivity of the slurry as an indication of solids concentration. In these tests, a pair of electrodes were mounted in diametrically opposed relation to one another to measure the conductivity of the slurry across the full diameter of the stream. It was stated that electrode design and positioning is critical and that electrodes mounted flush with the wall of the container vessel interfere with electrical field flux and give rise to poor reproducibility of results from calibrations. Difficulties in generating an attenuating signal which can pass through a large diameter pipe containing a high concentration slurry was not addressed.
It is known that the concentration of solids in a slurry moving in a pipeline is not always uniform throughout the cross-section of the pipe. For example, in horizontal pipes with solids having specific gravity greater than one, and particularly in long horizontal runs of pipe, a stratification of solids will develop from the top to the bottom of the pipe, with a minimum of solids at the top and a maximum concentration near the bottom.
It is also known that the conductivity of the conveying fluid may vary in a commercial slurry transport system such as used in a phosphate mining and processing plant. For example, in a water slurry of phosphate matrix, the amount of soluble chemicals in the matrix may vary substantially, with the result that ionization and consequently the conductivity of the fluid will not always remain constant even though the composition of the water used to mix the slurry is unchanged. Thus, in a concentration sensor utilizing conductivity as a measure of concentration, variations in conductivity of the liquid phase of the slurry will adversely affect the reliability of the indicated concentration. It is, accordingly, a primary object of the present invention to provide an improved on-line sensor for sensing and indicating the concentration or percent of solids moving in a large slurry pipeline.
Another object is to provide an improved method of and apparatus for providing an accurate and instantaneous indication of the slurry concentration at or adjacent the entrance to the pipeline, and for providing a feedback for controlling the solids in the slurry to near the optimum concentration.
An additional object is to provide a method of and apparatus for measuring a slurry concentration, utilizing the conductivity of the slurry as an instantaneous indicator of concentration.
A further object is to provide such a slurry concentration measurement system utilizing a plurality of sensing electrodes spaced around the pipeline in an annular array for measuring the conductivity of the slurry.
Yet another object is to provide such a system which is not affected by erosion resulting from the abrasive slurry moving through the pipeline.
A still further object is to provide such a system including means for continuously monitoring the conductivity of the liquid phase of the slurry, and for adjusting the calibration of the system to compensate for variations in the liquid conductivity.
It is another object to provide a slurry concentration sensor which will provide an average volumetric percent solids measurement of the slurry, and provide an instantaneous readout or display of the concentration measurement which may be utilized to control the mixing system and thereby the solids concentration entering the pipeline.